Electrical system



Aug. 29, 1939. A. F. PRICE ELECTRICAL SYSTEM Filed May 5, 1936 3/ ourparPLATE SUPPLY d. C.FILAMENT SUPPLY INPU T FIG? 3 OUTPUT VSUPPLY L1A.C.FILAHENT S'UPPLY INVE N TOR AFPR/CE 8! 04m 6. M

A TTORNEY Patented Aug. 29, 1939 UNITED STATES PATENT OFFlCE BellTelephone Laboratories,

Incorporated,

New York, N. Y., a corporation of New York Application May 5, 1936,Serial No. 77,986

6 Claims. (01. 179-171) This invention relates to electrical systemsemploying space discharge devices, and, more particularly, to suchsystems in which it is desired to maintain a substantially constantpotential on the control electrode'of such devices,

An object of the invention is to'simplify and reduce the cost of suchsystems.

A feature of this invention comprises the combination of a spacedischargedevice having a controlelectrode and a resistor witha symmetricnon linear voltage-current characteristic associated with it andtraversed by a variable unidirectional current for maintainingsubstantially constant the bias on the control electrode.

Another feature comprises the combination of such a device and resistorin which the latter is of a material having a substantiallyinstantaneously-acting negative resistance-current characteristicsubstantially independent of tempera-j ture;

Still another feature comprises the combination.

which follows in conjunction with the appended drawing wherein:

Fig. 1 shows an electrical system, more specifically, an audio-frequencyamplifier of the push-pull type, embodying the invention; and

Fig. 2 shows another electrical system embody ing a modification of theinvention.

Referring to Fig. 1, incoming line In is connected to the primarywinding ll of a transformer l2 having a secondary winding l3 inductivelycoupled to the primary and provided with a center tap l4. Each end ofthe secondary is connected through a conductor 16 to the controlelectrode or grid ll of -an electrical discharge device or vacuum tubel8. The center tap I4 is connected through conductor IS with thenegative terminal of the plate voltage supply, which may be a battery orother source of direct current voltage. The filaments or cathodes 20 ofthe tubes are connected with secondary windings 2| of a transformer 22,the primary windings 23 being serially connected and connected to asource of alternating current. Each plate or anode 24 is connectedthrough conductors 25 with an end of the primary Winding 26 of outputtransformer 21, the winding having a center tap 28 connected throughconductor 29 with the positive terminal of the plate supply. A secondarywinding 30 is inductively coupled to the winding 26 and its ends areconnected with outgoing line 31. Connected to the mid-points 32 ofcathode transformer windings 21 through conductors 33, 34, is a resistoror variable resistance device 35 of a material known commercially asthyrite. Thyrite is a composition of silicon carbide, graphite and clay,is described in K. McEachron Patent ,822, 42, issued September 8, 1931,and has a substantially instantaneously acting negativeresistance-current characteristic, the voltage-current characteristicbeing symmetric and non-linear and substantially independent oftemperature. The resistor is connected through conductor 36 withconductor l9. It is apparent that the resistor is connected in thecommon return path for the control electrode-cathode circuit and theanode-cathode circuit and that the variable unidirectional plate currenttraverses it.

In amplifiers in which the grids are biased almost down to cut-off toobtain more power from agiven size vacuum tube than can otherwise beobtained, a separate source of grid bias potential has been employed,usually, a rectifier. This has been considered necessary because theratio of full load to no load plate current in such amplifiers isapproximately 3 to 1 and, consequently,

a sufficiently constant grid bias could not be obtained from the'voltagedrop across a resistance in the cathode return circuit as is the commonpractice in ClassA .amplifiers.

If, however, such a grid bias resistance is replaced by a'resistor whoseresistance varies with current, flow therethrough such that the voltagedrop across it remains substantially constant with a 3 to 1 change incurrent therethrough, the grid bias potential may be obtained from thatsource and the separate source of grid potential eliminated. This ismade possible by the use of the resistor 35, the drop across it beingsubstantially constant for load current fluctuations of the statedrange.

The current-voltage characteristic of thyrite is expressed bytheequation:

I =lcV" where I=current.

k: a constant.

n is an exponent which depends upon the mixture and manufacturingprocess.

This variable resistance material is obtainable commercially with anexponent n equal to 4 and similar material can be made having anexponent n equal to 5+. For a 3 to 1 current change, therefore, it isapparent that, with n=4=, the corresponding voltage change Will be only1.32 to 1, and with an exponent of n=5, only 1.25 to 1.

By putting more than one resistor in cascade, the voltage variation maybe reduced even more. The electrical system of Fig. 2 is substantiallythe same as that of Fig. 1, like parts having corresponding identifyingnumerals. the member 35, however, there is provided a second variableresistance device or resistor 31 and a linear current-voltagecharacteristic or ohmic resistance 38 in the common return circuit, the

conductor 19 being connected at a point intermediate the members 31, 38,the resistance of the resistor 35 being small compared to the combinedresistance of the members 31, 38.

If each of members 35, 31 has the characteristic I =kV and the currentvaries in the ratio of 3 to 1, the voltage drop across the member 35will vary in the ratio of 1.32 to 1. If the resistance of member 38 islarge compared to that of member 31, the current variation throughmembers 31, 38 will approach 1.32 to 1 and the voltage variation acrossmember 31 will aproach 1.073 to 1 as a maximum. A substantial saving inthe cost of high power amplifier equipment is made possible by thedescribed arrangements.

While this invention has'been disclosed with reference to a specificembodiment, it is to be understood that it is not limited thereto but bythe scope of the appended claims only.

What is claimed is:

1. The combination of a space discharge device comprising a cathode, ananode and a control electrode, a cathode-anode circuit, acathode-control electrode circuit, non-ohmic resistance means common tosaid circuits across which the potential drop increases only slightlybetween no load and full load current, and additional resistance meanscommon to said circuits and comprising an ohmic and a non-ohmicresistance portion for reducing the effects on the control electrode ofsuch increases in potential drop across said first-mentioned means, saidnon-ohmic resistances each having a negative resistance-currentcharacteristic.

2. In combination, a space discharge device comprising a cathode, ananode and a control electrode, a cathode-anode circuit, a cathodecontrolelectrode circuit, and means common to said circuits for applying asubstantially constant potential on said control electrode for widevariations in current fiow in the cathode-anode circuit, said meansincluding members of a substantially instantaneously-acting negativeresistance-current characteristic resistance material, one of saidmembers being in parallel with a series combination of the other memberand an ohmic resistance.

3. In combination, a space discharge device comprising a cathode, ananode and a control electrode, a cathode-anode circuit, a cathode- Inaddition to control electrode circuit, and means common to said circuitsfor applying a substantially constant potential on said controlelectrode for wide variations in current flow in the cathode-anodecircuit, said means including members of a substantiallyinstantaneously-acting negative resistance-current characteristicresistance material, one of said members being in parallel with a seriescombination of the other member and an ohmic resistance, the resistanceof the combination being relatively great compared to that of said onemember.

4. In combination, a space discharge device comprising a cathode, ananode and a control electrode, a cathode-anode circuit, a cathodecontrolelectrode circuit, and means common to said circuits for applying asubstantially constant potential on said control electrode for Widevariations in current flow in the cathode-anode circuit, said meansincluding members of a substantially instantaneously-acting negativeresistance-current characteristic resistance material, one of saidmembers being in parallel with a series combination of the other memberand an ohmic resistance, the resistance of the combination beingrelatively great compared to that of said one member, connection to thecontrol electrode being made through a conductor in the cathode-controlelectrode circuit that is connected intermediate the member and theresistance of the series combination.

5. In combination a space-discharge device comprising a cathode, ananode and a. control electrode, a cathode-anode circuit, acathodecontrol electrode circuit, non-ohmic resistance means common tosaid circuits, and additional means, a part only of which comprisesnon-ohmic resistance means in shunt with said first-mentioned means formaintaining said biasing potential substantially constant with saidcathodeanode current variation, the non-ohmic resistance means eachhaving a negative resistance-current characteristic.

6. The combination of a space discharge device comprising a cathode, ananode and a control electrode, a cathode-anode circuit, a cathodecontrolelectrode circuit, negative resistance-current characteristic resistancemeans common to said circuits capable of supplying a potential varyingonly slightly with cathode-anode current variations, and additionalmeans, a part only of which includes negative resistance-currentcharacteristic resistance means, in shunt with said first-mentionedmeans, said additional means applying to said control electrode abiasing potential that remains substantially constant with variation invalue of current flow between no load and full load current in saidcathode-anode circuit.

ARNOLD F. PRICE.

